Abstract: First and second loop coils are located at respective positions separate from each other by a predetermined distance. A first electric signal is generated which depends on an inductance of the first loop coil. A second electric signal is generated which depends on an inductance of the second loop coil. Detection is made as to a speed of a vehicle, which passes through first and second given regions determined by the respective positions of the first and second loop coils, in response to the first and second electric signals. The first electric signal is compared with a reference signal to decide whether or not a vehicle passes through the first given region determined by the position of the first loop coil. The reference signal is controlled in response to conditions of the first electric signal which occur in a follow-up time. The follow-up time is changed in response to the detected vehicle speed.